Directly modulated light emitting diodes are limited to relatively low modulation bandwidths. This is because the spontaneous optical emission is limited by the recombination lifetime of the injected minority carriers, which is generally greater than ins. Although theory predicts that this lifetime can be decreased through Purcell enhancement, no such demonstration has shown a speed more than an order of magnitude in the visible spectrum. Semiconductor laser diodes can achieve higher modulation bandwidths because of the speed of stimulated emission, and the laser diodes tend to be limited by the interplay of many factors including an RC rise time, photon lifetime, and the gain dynamics of the material. Through years of engineering advances, commercial laser diode sources have achieved direct modulation bandwidths of approximately 10 Gb/s. Because of this limitation, costly arrays of laser diodes, drivers, external modulators, and wavelength division multiplexing optics are used for requisite data rates. Thus, the telecom market has a serious need for a directly modulated optical source with modulation bandwidths beyond the current 10 Gb/s limit.
In addition to the need for high-speed direct modulation, other emerging applications in spectroscopy and lab-on-a-chip designs require broadband and tunable optical sources for metrology applications. Again, laser diodes and light emitting diodes fall short, with tunable bandwidths typically on the order of 50 nm at most. There is demand for an optical source, which can be tuned over hundreds of nanometers with a simple electrical drive and without moving parts.
In addition, there is a need for very small optical sources. The rapid growth in demand for optical communication capabilities has created a renewed interest in nano-scale optical sources. The compelling need for compact, high-speed communications between cores at the chip and board level further punctuates the need for integrated optical sources and modulators. Creating an optical emitter with the functionality to be modulated at greater than 100 GHz frequencies will revolutionize electronic devices and data communications systems. However, generating light in silicon has proven to be very difficult. Because silicon is an indirect bandgap semiconductor, band-to-band, electron-hole radiative recombination is inefficient. For this reason, light sources for optoelectronic systems are typically implemented using direct-bandgap compound semiconductor devices with quantum well or quantum dot active regions that provide the optical gain necessary for laser operation. These sources are off-chip, made from non-CMOS materials, and have size scales that are several orders of magnitude larger than the transistor. Integrating compound semiconductor light sources with silicon photonic and microelectronic systems is complicated and costly.
Approaches for light emission in silicon have been proposed and include silicon nanocrystals as taught by Pavesi, L., et al., in “Optical gain in silicon nanocrystals”, Nature, vol. 408, 440-444, 2000, include Si/SiO2 superlattices as taught by D. J. Lockwood, et al., in “Quantum confined luminescence in Si/SiO2 superlattices”, Phys. Rev. Lett., vol. 76, 539-541, 1996, include doping with active impurities, such as erbium, as taught by H. S. Han, et al. in “Optical Gain at 1.54 μm in Erbium-doped Nanocluster Sensitized Waveguide” Appl. Phys. Lett., vol. 79, 4568-4570, 2001, and include stimulated Raman scattering as taught by R. D. Claps, et al., in “Observation of Raman Emission in Silicon Waveguides at 1.54 μm”, Opt. Express, vol. 10, 1305-1313, 2002. These demonstrations were all accomplished with optical excitation from an external light source and may not be suitable for efficient on-chip light emission. Porous silicon light emitting diodes that might be suitable for on-chip integration with electronics have been demonstrated as taught by P. Fauchet, in “The integration of Nanoscale Porous Silicon Light Emitters” in Jour. of Luminescence, vol. 80, 53-64, 1999. But, the direct electrical to optical conversion efficiency of these devices is at best a fraction of 1.0% as taught by H. Wong et al., in “Silicon Integrated Photonics Begins to Revolutionize.” Microelectronics Reliability, vol. 47, 1-10, 2007.
A first experimental demonstration of light emission due to inelastic electron tunneling was taught by J. Lambe et al., in “Light Emission from Inelastic Electron Tunneling”, Physical Review Letters, vol. 37, no. 14, pp. 923-925, 1976. In their original device, a millimeter-sized square of Aluminum was oxidized, creating 3 nm of alumina oxide. A counter electrode of silver and gold was then electron-beam evaporated on top of the alumina, creating a metal-insulator-metal structure. The insulator is sufficiently thin to allow for non-negligible tunnel current across the gap. When voltage was applied across the electrodes, visible light was emitted from the device, with a cut-off such that hfp=eV, where h is Plank's constant, fp is the cut-off frequency, e is the electron charge, and V is the applied voltage. This optical emission was broadband, highly incoherent, and not very directional, much like an incandescent source, such as a light bulb. It appears that simply changing the voltage changed the color of the emission, thus, there was the creation of a solid-state and very broad-band tunable emitter with threshold voltages on the order of 1V. While this was a step forward for optical devices, the problems lie in the efficiency of the device, which was estimated at one collected photon for every 105 electrons of current. Because of this severe performance limitation, the devices are deemed practically unsuitable for commercial exploitation. As suggested in the original work, and as theoretically shown in a subsequent publication by L. C. Davis as taught in “Theory of Surface-Plasmon Excitation in Metal-Insulator-Metal Tunnel Junctions,” Physical Review B, vol. 16, no. 6, pp. 2482-2490, 1977, the mechanism of optical generation in the junction is inelastic electron tunneling, which is the physical principle behind conversion of tunnel current into light.
As the electron tunnels through the insulator, the electronics create an optical excitation through this scattering process. This is to be contrasted with electron injection, in which the high energy electron would tunnel across the barrier and spontaneously decay to the thermal level through optical emission. In the inelastic tunneling process, it has been shown that the efficiency at optical frequencies is on the order of 10% and no free space optical modes can fit into a 3 nm spacing. As an application of Maxwell's equations will show, the only available optical modes are surface plasmon resonances in this structure. It was the surface plasmon resonances that caused the inefficiency of the device. Surface plasmon modes in such a structure have propagation lengths on the order of tens of nanometers, and hence, a good deal of the optical excitation was immediately lost to absorption via Joule heating. For the metal-insulator-metal geometry consisting of a 2 mm2 device area having a bottom aluminum layer, a middle aluminum oxide insulating Al2O3 layer, and a top silver layer, only plasmons that are able to evanescently tunnel through the top contact and scatter from surface roughness can become free space light. The optical modes experience a large impedance mismatch as well as the mode-size mismatch. Thus, the device does not have a well-designed plasmonic out-coupling means to achieve improved efficiency.
While the physical control of nanostructures was beyond the reach of earlier fabrication techniques, modern technology makes such plasmonic out-coupling entirely possible. A doctoral dissertation of Josh A. Conway was on the subject of efficient optical coupling to a nano-scale gap as taught in “Efficient Optical Coupling to the Nanoscale,” Doctoral Dissertation, UCLA, 2006. In this work, it was found that electromagnetic energy could be coupled from a free-space field into a metal-insulator-metal volume of tens of nanometers with improved efficiency. However, a 35 dB of loss is encountered using 90 degree angles of a silver contact relative to the aluminum oxide and aluminum substrate.
Another key aspect of inelastic electron tunneling is its speed. This is exemplified by the large 1 eV optical bandwidth. From Heisenberg's Uncertainty Principle, it is known that such large bandwidths mean that the inelastic tunneling is a tremendously fast process, on the order of 10−15s=1 fs. Conventional light emitting diodes are principally limited by the lifetime of the injected minority carriers as taught by B. E. A. Saleh et al., Fundamentals of Photonics, p. 606, John Wiley and Sons, New York, 1991. The lifetime of injected minority carriers tend to be on the order of ins to 50 ns. This means that the inelastic tunnel junction has a fundamental modulation limit, which is five orders of magnitude faster than the standard art. However, the velocity of an electron on the Fermi surface is 1.4×106 m/s for both Ag and Au as taught by C. Kittel in “Introduction to Solid State Physics”, p 150, John Wiley and Sons, New York, Seventh Edition, 1996. The thickness of the gap is on the order of 1 nm=10−9 m. As such, the expected speed of an electron tunneling across the gap to be on the order of 10−15 s, which is 1 fs. This rapid process, then, enables extremely fast and directly modulated optical source, unlike any conventional emitter.
The prior art includes U.S. Pat. No. 4,163,920 entitled “Solid State Source of Radiant Energy having a Controllable Frequency Spectra Characteristic”, by Lambe et al., issued Aug. 7, 1979, U.S. Pat. No. 7,010,183, entitled “Surface Plasmon Devices”, by Estes, et al., issued Mar. 7, 2006, and U.S. Pat. No. 7,177,515 entitled “Surface Plasmon Devices”, by Estes, et al., issued Feb. 13, 2007. All of these devices use large area structures to generate plasmons in the tunnel junction. By the fundamental laws of causality, these devices are far too inefficient to ever be useful. These and other disadvantages are solved or reduced using the invention.